[v0.2.0] Audit and replace any remaining time.Sleep in tests

[neverDefined]

Problem

time.Sleep in tests is a flake source. The shared-anvil pattern already avoids it for the main flow, but there may be stragglers (especially around concurrency and mempool waits).

Proposal

1. git grep 'time\.Sleep' -- '*_test.go' and audit each hit.
2. Replace each with one of:
   • WaitForBlock / WaitForMemPoolEmpty (poll loops with a deadline)
   • A t.Context()-aware select on a channel
   • require.Eventually (testify) for "X is true within Y"
3. Where a small delay is actually needed (e.g. between two impersonate calls), document why with a comment.

Acceptance criteria

• git grep 'time\.Sleep' -- '*_test.go' is empty, or every remaining hit has a // reason: comment.
• go test -race -count=10 ./... passes (10× catches flakes).